Full optical devices implementing correlation functions between light beams are required in the field of information processing by optical techniques.
Such operations are particularly useful whenever automatic image recognition by comparison with a much larger image or with a series of images is required. In this way a particular geographical area can be identified within a wide terrestrial celestial area, the presence of a word in a text can be detected, et cetera. In these cases the comparison is generally made between two optical beasm whose intensity is spatially modulated by the respective images or by their Fourier Transforms and the recognition takes place in correspondence with the maximum of a correlation function detected by a suitable analyzer.
Also in the domain of information processing by optical computers logical operations are to be effected between binary digit matrices transferred on optical beams, wherein the presence of absence of light at different points is associated with a different logic level. By spatial and/or time correlation between two beams of its kind, the known logic function AND, OR etc can be obtained, while operating at the same time on a great number of parallel binary digits, thereby obtaining a large processing capacity.
Devices capable of carrying out the correlation between light beams are now well known. These include the so-called LCLV (Liquid Crystal Light Valve) described in the paper "Digital Optical Computing" by Alexander A. Sawchuk et al., issued in Proceedings of the IEEE, vol. 72, No. 7, July 1984. These devices consist of a layer of semiconducting material CdS, forming a photoconducting surface, and of a liquid crystal surface, separated by an opaque layer, by a reflecting layer and by an insulating layer. Two external transparent and electroconducting surfaces allow the application of an a.c. biasing voltage. A light beam incident on the photoconducting surface causes a variation of the input impedance of this layer at different points proportionally to spatial light intensity distribution, and hence a variation of the voltage drop across the adjacent liquid crystal layer, thus obtaining a corresponding variation of crystal orientation. By sending towards the liquid crystal surface a polarized light beam, a reflected beam is obtained with polarization varying at different points as a function of the liquid crystal rotation angle. If the reflected beam is examined by a suitable polarization analyzer, an intensity-modulated beam is obtained according to a beam incident onto the photoconducting surface. However, this device has a number of limitations due both to the electro-optical conversion operated between the different layers, and to the use of liquid crystals. In fact the maximum spatial resolution is about 40 lines/mm and the response time is of 30 ms.